1. Field of the Invention
This invention relates to RF micro-electro-mechanical system (MEMS) capacitive switches and, more particularly, to the reduction of trapped charge in RF MEMS capacitive switches.
2. Description of the Related Art
A radio frequency (RF) micro-electro-mechanical system (MEMS) capacitive switch includes a top electrode that is displaced toward a bottom electrode in response to the application of a voltage differential between the electrodes. An RF signal applied to one of the electrodes sees a variable capacitance based on the displacement. In various types of MEMS capacitive switches the top electrode may include a flexible membrane that is suspended between two or more posts and displaced parallel to the bottom electrode, a rigid beam that is cantilevered from a single post or a flexible vertical beam that is incrementally displaced to a horizontal position akin to a “zipper”. The top electrode exhibits a resilience that resists the displacement and urges the top electrode to return to a deactuated position, which it does when the voltage differential is removed. Different types of MEMS switches may be “binary” such as the membrane or cantilevered switches or “analog” such as the zipper switch.
To both maximize the capacitance in the actuated state and to prevent the top electrode from contacting the bottom electrode, the MEMS capacitive switch includes dielectric material formed on the bottom electrode. One problem is that, when the top electrode is displaced and contacting the dielectric material in the actuated state of the switch, electric charge can tunnel into and become trapped in the dielectric material. As a result, and due to long recombination times in the dielectric, the amount of this trapped charge in the dielectric material increases progressively over time and exerts a progressively increasing attractive force on the top electrode. When the top electrode is in its actuated position, this attractive force tends to resist movement of the top electrode away from its actuated position toward its deactuated position. The amount of trapped charge can eventually increase to the point where the attractive force exerted on the top electrode by the trapped charge is in excess of the inherent resilient force of the top electrode, which is urging the top electrode to return to its deactuated position. As a result, the top electrode becomes trapped in its actuated position, and the switch is no longer capable of carrying out a switching function. This is considered a failure of the switch, and is associated with an undesirably short operational lifetime for the switch.
Many prior attempts have been made to solve or at least reduce the dielectric charging problem. One approach was to change the properties of the dielectric material so as to modify the extent to which the dielectric material is “leaky”. Another prior approach is to alter the waveform used for the DC bias voltage. Another prior approach is to “texture” one or both of the top electrode or dielectric material. Yet another prior approach is to pattern the dielectric material to form an array of posts. This approach reduces the amount of trapped charge but also reduces the amount of dielectric material between the electrodes, which runs counter to the traditional design goal to maximize the capacitance ratio of the switch.
Referring now to FIGS. 1a-1d, an embodiment of an RF MEMS capacitive switch 10 of a “membrane” type is shown in which the dielectric material has been patterned to form an array of dielectric posts 12 that separate a bottom electrode 14 from a suspended top electrode 16. In this embodiment, the membrane itself is formed of a conductive material such as aluminum that forms top electrode 16. An RF signal is applied to one of the bottom electrode and the membrane. A number of vent holes 18 are etched in the membrane to facilitate removal of sacrificial layers used during fabrication and to reduce squeeze-film damping when the membrane is displaced. The vent holes 18 in the membrane are placed away from the underlying posts 12 to ensure complete metal/dielectric coverage 20 in the actuated state to maximize the capacitance. As shown, when top electrode 16 is contacting the dielectric post 12 in the actuated state, electric charge 22 can tunnel into and become trapped in the post. The problem of trapped charge remains but is reduced proportional to the sparsity or fill-factor of the posts as compared to a solid dielectric layer.